Field of the Invention
The present invention relates to an ink set and an ink jet recording method using the ink set.
Description of the Related Art
In recent years, an ink jet recording method enables the recording of images having high definition and high color developability that are almost equivalent to those achieved by silver halide photography and offset printing. As the inks used for the ink jet recording method, inks containing pigments as coloring materials (pigment inks) have been widely used in order to record images having higher toughness including gas resistance and light resistance. However, the images recorded with the pigment inks on glossy papers unfortunately have lower color developability than those of images recorded with dye inks.
In particular, monochrome images recorded with the pigment ink emit scattering light due to unevenness on the surface of a pigment layer or gaps in a pigment layer as reflected light by the images, and the light is visually observed as white light. This unfortunately reduces the black density in appearance and impairs the color developability. In addition, images recorded with the pigment ink cause what is called a bronzing phenomenon that light having a different color from that of incident light is reflected. For example, images recorded with a pigment ink containing carbon black as the coloring material are likely to cause a yellowish bronzing phenomenon, which is the cause of impairing the black density in appearance.
To solve these problems, it has been tried to improve the glossiness and the color developability of images to be recorded and to suppress the bronzing phenomenon by combination use of an ink containing no coloring material (what is called a clear ink) and a pigment ink. For example, in order to improve the glossiness and to suppress the bronzing phenomenon of images recorded with a pigment ink, an ink set including a clear ink containing a water-soluble resin has been disclosed (Japanese Patent Application Laid-Open No. 2009-197211). Separately, an ink set that includes a clear ink containing a polyisocyanate compound and a pigment ink containing a polyurethane compound or the polyisocyanate compound has been disclosed (Japanese Patent Application Laid-Open No. 2011-195610). The ink set can improve the color developability of images and can suppress bleeding. Another ink set that includes a clear ink containing a resin particle and a plurality of pigment inks containing carbon black at various concentrations have also been disclosed (Japanese Patent Application Laid-Open No. 2005-052984). The ink set can reduce the gloss unevenness of images.
The inventors of the present invention have studied the color developability and the bronzing resistance of images recorded with the conventional clear inks described above and the pigment inks containing carbon black as the coloring material. The result has revealed that the effects of improving the color developability and the bronzing resistance of images are observed but are not achieved at high levels by using any of the clear inks.
During the above study, a new problem commonly caused when conventional ink sets are used has been found. Specifically, when a clear ink is applied onto the images recorded with a pigment ink containing carbon black as the coloring material, images colored with various hues are recorded depending on application amounts of the clear ink, and this coloring differs from the bronzing phenomenon. On this account, such a problem that the black density in appearance of images recorded by using a conventional clear ink and a pigment ink containing carbon black as the coloring material is impaired has been found. This coloring is thought to be caused by fringe between reflected light from the surface of a “pigment layer” constituting an image and reflected light from the surface of a “resin layer” formed on the “pigment layer” by a resin in the clear ink. Hereinafter, such reflected light coloring generated by the fringe between reflected light from the surface of a resin layer and reflected light from the surface of a pigment layer is called “coloring by fringe”.
The problem of “coloring by fringe” caused by using conventional clear inks is not recognized in Japanese Patent Application Laid-Open No. 2009-197211, Japanese Patent Application Laid-Open No. 2011-195610 and Japanese Patent Application Laid-Open No. 2005-052984. In other words, the “coloring by fringe” has not drawn attention and is a novel problem. When the ink sets disclosed in Japanese Patent Application Laid-Open No. 2009-197211, Japanese Patent Application Laid-Open No. 2011-195610 and Japanese Patent Application Laid-Open No. 2005-052984 are used, the improvement effect of the color developability and the suppressive effect of bronzing phenomenon are observed to some extent but are insufficient, and the coloring by fringe is caused on images recorded. In other words, the black density in appearance of the recorded black images fails to achieve a satisfactory level.
An object of the present invention is thus to provide an ink set enabling the recording of images that have excellent color developability and bronzing resistance and have suppressed a coloring by fringe when a clear ink and a pigment ink are used in combination. Another object of the present invention is to provide an ink jet recording method using the ink set.